Air fractionating cycle and apparatus



6 c. J. SCHILLING AIR FRACTIONATING CYCLE AND APPARATUS Original FiledOct. 9, 1946 mdE 0. J. SGHILLING Ina/enfor- Ammu Un swe P r AIRFRACTIONATIN G CYCLE AND APPARATUS Clarence .l. schilling Allentown,Pa., assignor to Air Products Incorporated, a corporation of MichiganOriginal application October 9, 1946, Serial No. 702,112, now Patent No.2,620,637, dated December 9, 1952. Divided and this application April16, 1952, Serial No. 282,644

11 claims. (cl. 62-122) This invention relates to the fractionation ofair for the separation of oxygen and nitrogen in. states of substantialpurity.

This application is a divisionof application, Serial No. 702,112, filedOctober 9, 1946, now Patent No. 2,620,637, granted December 9, 1952 forAir Fractionating Cycle and Apparatus. 1

An objective of the invention is to provide an air fractionating cyclewhich is substantially self-regulating and which requires the minimum ofoperator attention.

An objective of the invention is to provide an air fractionating cyclecapable of being put into operation in the minimum of time, permittingshutdowns at desired intervals without material loss of time inrestarting.

An objective of the invention is to provide an air fractionatingapparatus which may be assembled in a very small space for readyportability.

These and other advantages are attained by certain modifications of theconventional double column apparatus and cycle which will be describedin detail with reference to the attached drawings, in which Fig. 1 is adiagram and flowsheet of the operating cycle, in which apparatuselements are indicated by conventional symbols;

Fig. 2 is a detail, in elevation of a choke coil which may be used toadvantage to controlthe flows of various feed streams, and r Fig. 3 is adetail, in section, of an orifice fitting which may be used in place ofthe choke coil for the same purpose.

Referring first to Fig. 1,? atmospheric air enters the system at throughan air filter 11 and is compressed to about 65 poundsgauge inthefirststage 12 of an air compressor 13. The low. pressure. air is brought backto atmospheric temperature in a water-cooled intercooler 14 andseparated from condensed water in a trap 15. The low pressure air streamthen passes through towers 16 and 17 in which it is scrubbedsuccessively with a weak and with a strong solution of a caustic alkalifor the re moval of the greater part of the carbon dioxide. These towersare operated. continuously, weak solution being withdrawn and strongsolution transferred and replaced at intervals. 1 r 1 The scrubbed air,which is substantially but not entirely free from carbon dioxide, passesthrough a water trap 18 to the second compression stage 19, which. ismain- .tainedat about 435 pounds gauge, thence through an intercooler 20and a trap 21 to the .final compression stage 22. During the startingperiod this stage is held gaseous fractionation products, as will bedescribed. The

liquid water produced by this cooling and carried as a mist in the airstream is trapped out in any preferred form of water separator 25 anddrained from the system. All of the water traps referred to arepreferably provided with automatic bleeder valves not shown.

The partially dehydrated air stream next passes through one of thedesiccators 26--26 in which the water content of the air is reduced to afigure which, in view of certain precautions to prevent stoppage by icecrystals, later to be described, is negligible. These desiccators, whichcontain contact beds of adsorbent material such as silica gel oractivated alumina, are used alternately in the well known manner (cf.lkeda, 1,541,147) one being in use while the other is being regeneratedby blowing with air supplied by a motorblower 27 and heated in anelement 28. The valve arrangements for these diversions are. well knownand are not shown.

The substantially dry air stream now returns to the primary interchanger24 where it is further cooled to about 253 K. in the second stage coil29. It then flows to an interchanger 30, an element in an externalrefrigeration cycle later to be described, in which it is cooled toabout 233 K. by heat interchange with a boiling liquid refrigerant, asfor example Freon 12 (dichlorodifiuoromethane).

The airstream then returns to third stage coil 31 of the primaryinterchanger, in which it is cooled by heat interchange with columnproducts to about 140 K., the air at this temperature and pressure beingstill in the gaseous phase.

The refrigerated air passes through a conduit 32 to a manuallycontrolled expansion valve .33 by which its pressure is reduced to aboutpounds gauge with a concommitant reduction of temperature to about 100K. The stream then passes to a point of division 34, a portion of thestream, as for example 40 percent, flowing through conduit 35 into thebase of the high pressure column 36, the remainder passing on to a lowpressure column through various steps of heat interchange laterdescribed. he proportioning of the two divisions of the air stream iscontrolled by the balancing of flow resistances interposed in theseveralbranches.

In the high pressure column, which may be packed or provided with bubbleplates as preferred, the portion of the total air feed entering at 35 isseparated by fractionation into an oxygen-rich liquid (crude oxygen),which collects in a pool 37 in the base of the column, and a vaporfraction consisting substantially of nitrogen which is delivered fromthe top of the column at 38.

r Theycrude oxygen passes from pool 37 through a choke coil 39 whichfunctions, in lieu of an expansion valve, to reduce the pressure onthestream to about 10 pounds gauge, the temperature falling to about 83 K.As this temperature is below the boiling point of nitrogen at thepressure existing in the upper end of the high pressure column, thepassage of the expanded crude oxygen through coil .40, immersed innitrogen vapor, provides by condensation a liberal supply of refluxliquid for the packing or pl ates below. I

r The gaseous nitrogen from the high pressure column passes viaconduit..41 to a coil 42 immersed in a pool of liquid oxygen boiling inthe base; of .the low pressure column 43. Condensing inthis coil, itpasses by way of conduit 44 to a coil 45 in secondary interclianger .46.this coil it is cooled by heat interchange with the feed of expandedcrude oxygen to t he lowprcssure column. It is finally passed through achoke coil 47, by which its pressureis reduced to about 10 pounds gaugeand its temperature to about 811K and is fed into the low pressurecolumnat 48 as a liquidnitrogen stream. The entry into the column may bedirect, as in conventional practice, a i may be by waste s o a e t n 8.at de- The crude oxygen issuing trom coil 40 in the head of the highpressure column at about 83 K. passes via conduit 49 to the element 46in which it is in heat interchange with the liquid nitrogen abovedescribed and with a stream of liquid air described below. In thisinterchange a portion of the liquid of the crude oxygen stream isvaporized, and as the pressure has already been reduced (at 39) toapproximately that of the low pressure column, the partially liquidstream is passed to the column through conduit 50, entering at pointSlat about 83 K.

Returning 'to division point 34 on the main air feed line, the portionof the total feed remaining after the side stream is diverted throughconduit 35 is carried through conduit 52 to a coil 53 immersed inboiling oxygen in the base of low pressure column 43. In this coil it iscondensed at about 100 K. without material change in pressure. The airstream then passes through conduit 54 to a coil 55 in secondaryinterchan'ger 46, in which it is in heat interchange with the crudeoxygen stream as above stated and in which its temperature a is furtherreduced to about 85 K. It is then passed through a choke coil 56 bywhich its pressure is 'reduced to about pounds gauge and its temperatureto about 82 K. and is fed into the low pressure column at 57 in theliquid condition.

The low pressure column may be provided with the conventional bubbleplates or with packing, as may be preferred. This column completes thefractionation of the air, delivering nitrogen in gaseous form at 58 andcollecting commercially pure oxygen (about 99.5%) in a pool 59 in thebase of the column. This oxygen product may be withdrawn in liquid formfrom the pool but I prefer to withdraw it as a liquid stream from thelowermost plate in the column. I

The liquid oxygen stream passes through conduit '60 to an interchanger61 in which it is cooled to a temperature at least several degrees belowits boiling point at the existing pressure, in heat interchange with theentire stream of gaseous nitrogen flowing from the low pressure columnthrough conduit 62. The refrigerated 1 surrounding the cylinder of pump63, in which it functions to prevent gas locking in the manner describedin the copending application or 'Carl R. Anderson entitled Pump forLiquefied Gases, filed October 21, 1943 under Serial No. 507,091, newPatent No. 2,439,957. From this jacket 'it passes through conduit 68 tothe primary interchanger 24 in which it is in counterflow to the airfeed streams in coils 23, 29 and 31, and is discharged at 69 as a gas atsubstantially atmospheric temperature and pressure. a p

In the design of the plant above described, the pressure drops acrossehokeeoils 39 and 47, located respectively in the crude oxygen andliquid nitrogen feeds to the low pressure column, are so balancedagainst the pressure drop across choke coil 56, located in the liquidair feed to the low pressure column, that the total ai'r feed "isdivided between the two columns in the desired proportion. Onceestablished, this relation is thereafter maintained through variedregulation of the primary expansion valve 33 by which the pressure atdivision point 34 is controlled. As the choke coil itselt is an elementof small size and low cost it may readily be replaced by another ofdifferent flow resistance if it becomes desirable to alter theproportions 'fed to the two columns. The coils, later described, aresimple helices of season walled, metallic tubing, the flow resistance ofwhich for any given volume and weight of fluid may readily be calculatedand checked experimentally.

The external refrigerating cycle above referred to as cooling the airfeed stream taken from interchange coil 29 consists essentially of acompressor 70, a watercooled liquefying interchanger 71, a subcoolingunit 72 in which the water-condensed liquid is in interchange with vaporfrom the evaporator, an expansion valve or choke coil 73 and aninterchanging evaporator 74 containing the air-cooling coils 30. Thevapor of Freon (the preferred cooling medium) is compressed to about v110 pounds gauge, liquefied, subc'ooled and expanded to about 430 mm.absolute in the evaporator, the vapor produced by interchange with airbeing used to subcool the water-condensed liquid. This pressure relationproduces a temperature in the bath of boiling Freon of about 233" K.

Other refrigerating liquids, of suitable high boiling points, such asammonia or sulfur dioxide, may be substi'tuted, but the use of Freon isadvantageous by reason of its high molecular weight and correspondinglyhigh heat carrying capacity, permitting the use of a'relatively smallunit for any given refrigerating eitect.

Referringnow to Fig. 2, the choke coils indicated at 39, 4 7 and 56 maybe helices of, for example, half hard seamless copper tubing, of suchinternal diameter and such length as to transmit the desired quantity offluid with the desired drop in pressure.

The pr'oportioning of these coils is not critical but they should besufliciently heavy to avoid risk of flattening the tubing in winding ona mandrel: for example, the internal diameter may be more or less thanone-half of the external diameter. The pitch of the turns is unimportantbut may conveniently be about twice the tube diameter. The length of thecoil and its most desirable internal diameter will vary widely with thequantities and pressures involved and must be calculated for anyspecific case. It is highly desirable, however, to choose a boresufiiciently great to call for at least several feet of tubing toprovide the necessary flow resistance.

The ends of the coil may be connected into the con duit which theycontrol by the use of tubing fittings but it is preferable to use thereducing fitting shown at 75 in Fig. 2, in which 76 is a fragment oftheconduit. The large end 'of the reducer is placed over the end of theconduit, the end of the coil is inserted in the small end of the reducerand run into the end of the conduit, and both ends may then be solderedwithout danger of solder enteringthe bore of the choke coil. This typeof con n ection is readily broken by melting the solder at the large endof the fitting.

' The use of a choke coil in an apparatus and cycle of the typedescribed has important advantages over the use of the conventionalneedle type valve, Whether the stream be liquid, gaseous or in mixedphase.

The primary advantage is the constancy of proportioning of the twodivisions of the air stream, this, however, being an advantage sharedwith other types of fixed orifice. The second, which also is shared withthe fixed orifice illustrated in Fig. 3, is that a circular orifice ofany given effective area has a far greater minimum dimension than theannular opening between the needle and the seat in the conventionalexpansion valve, thus reducing the liability to stoppage by entrainedsolids.

The major advantage, which 'is particular to the coil type of orifice,is that as the flow path is highly extended (ordinarily 1000 or moretimes the diameter of the opening) the latter may have amuch greatercrosssect ional area, for any giveii duty, than is possible where theopening is formed in a plate or button and is of an immaterial length. I1 s Thus the coil type or choke has proven to be a complete insuranceagainst stoppage by ice and carbon dioxide crystals, to "which "exansion valves are highly subject stresses and which requires such valvesto bemanipulatedat frequent intervals to maintain their regulation. Theshort orifice, though far preferable to the valve, is still somewhatsubject to choking, particularly in very small sizes.

If the size of the apparatus be such as to call for relatively largeflow-regulating openings, it is permissible to substitute for the chokecoil a simple orifice, such as illustrated in Fig. 3. This fittingmay bein the form of a button or plate 77 of hard metal retained in a coppercollar 78 by fillets 79 of hard solder or brass. The actual orifice 80should be tapered at least several degrees and in placing the fittingthe larger end should be directed downstream.

It is often desirable to provide a source of supply of pure nitrogen,either liquid or gaseous, in an apparatus of this type the primarypurpose of which is to supply pure oxygen. In such cases, instead ofdelivering the liquid nitrogen from choke coil 47 directly into the lowpressure column, as is customary, it is delivered into a reservoir 81which fills with the liquid and overflows through a conduit 82 and avalve 83 into the column at 48.

Liquid nitrogen may be withdrawn from this reservoir through a drainpipe 85 having a control valve 84, the upper valve 83 being throttleddown if any back pressure is to be overcome.

To obtain pure nitrogen in gaseous form from the high pressure column, abranch 86 from the high pressure nitrogen vent 38 is provided, thisbranch communicating with a conduit 87 within interchanger 24 and inheat interchange with the entering air supply. This conduit is providedwith a vent 88 controlled by a valve 89 through which gaseous nitrogenat atmospheric temperature and at any pressure up to that of the highpressure column may be withdrawn.

As soon as the operation comes into balance after either of the valvemanipulations above described, nitrogen and oxygen, each at about 99.5%purity, may be withdrawn from the system simultaneously, with somereduction of the normal oxygen output due to the reduced quantity ofreflux liquid available for the low pressure column.

In the preferred embodiment, element 36 has been shown as a gas-liquidcontact column although, obviously, any suitable structure forfractionating a gaseous m1x ture by liquefying a portion may be used.

The cycle above described, while including several conventional steps,differs from both the single column and the double column operations ofthe prior art in important particulars.

It resembles the single column in simplicity and in having but a singlemanual valve, but differs from it in producing considerably more oxygenof high purity than can be obtained from a single column of equaldimensions.

It differs from double column operation in taking only part of the totalair feed through the high pressure stage, thus materially reducing thedimensions of this element of the apparatus.

It diifers from conventional double column operation in producing theliquid nitrogen required for refluxing the high pressure column byinterchange with expanded crude oxygen rather than with the pure oxygenboiling in the low pressure column. This makes possible the thermal andthe physical separation of the columns, which, in turn, reduces theheight of the apparatus and makes for a very compact assembly.

It differs from both of the conventional types in having three liquidfeeds to the low pressure column, whereas the double column has two andthe single column but one. As these feeds are of widely differentcompositions and are at controllable temperatures it is possible tointroduce each at its equilibrium level in the column and to produce theoptimum temperature gradient and the optimum relation between liquor andvapor throughout the length of the column.

It differs from conventional operations in bringing to the singlemanually operated expansion valve a highly refrigerated air stream inthe wholly gaseous state, avoiding the difficulties incident toregulating a manual valve handling liquid or partly liquefied air, aswell as the risk of choking.

It differs from the conventional two stage operation in using flowresistances of fixed values in place of expansion valves in reducing theproducts of the higher pressure stage to the lower pressure, therebyavoiding the fluctuations in operating conditions, incident to manualcontrol of these flows and the risk of stoppages incident tothe use ofvalves at these locations.

The use of the auxiliary (Freon) refrigerating cycle assists in quickstarting, compensates the losses of refrigeration incident to adsorptionof water from a partially refrigerated air feed and permits the wateradsorption step to be conducted at the low temperature at which it ismost efiective.

It will be understood that the temperatures and pressures recited hereinare intended solely to be illustrative of preferred operating conditionsand that they are not limiting. The major advantages of the inventionare retained even though these conditions be departed from ratherwidely. It will also be understood that certain of the steps aboverecited may advantageously be used in combinations other than those inwhich they are used herein.

I claim:

1. In an air fractionating apparatus having fractionating columnsarranged to operate respectively at relatively high and relatively lowpressure: means: for liquefying at the higher pressure a stream ofgaseous nitrogen drawn from the high pressure column; means forexpanding to the lower pressure a stream of crude oxygen drawn from thehigh pressure column; means for subcooling said liquid nitrogen streamby heat interchange with said expanded crude oxygen stream; means forexpanding said subcooled nitrogen to the lower pressure; transfer meansincluding a path through a liquid receiving reservoir communicating withthe last-named expansion means and the low pressure. column fordelivering expanded subcooled nitrogen to the low pressure column, andvalve controlled means for withdrawing desired quantities of liquidnitrogen from said reservoir.

2. Apparatus for fractionating a mixture of gases comprising means forexpanding a stream of compressed and refrigerated gaseous mixture, meansfor dividing the expanded stream of compressed and refrigerated gaseousmixture into two substreams, first gas-liquid separation means forseparating one substream to produce two product streams, fractionatingmeans, separate conduit means for conducting each of the two productstreams and the second substream to the fractionating means, a flowresistance of fixed and predetermined value interposed in each of theseparate conduit means, the respective values of the flow resistancesbeing so proportioned as to produce a desired volumetric relationshipbetween the two substreams.

3. Apparatus as described in claim 2, in which each flow resistance isan orifice in elongated tubular form and of relatively small bore.

4. In the separation of gaseous mixtures including a relatively highboiling constituent as an impurity, in which a compressed andrefrigerated stream of the gaseous mixture in vaporous form isfractionated into a liquid product and a gaseous product, the stepscomprising reducing the pressure of the stream of the compressed andrefrigerated gaseous mixture by a controlled amount so as not to depositsubstantial impurity out of the mixture during such pressure reduction,then liquefying a stream of'the mixture and depositing out impurity insolid form, expanding. a stream of liquefied mixture by restricting aportion of the stream of liquefied mixture to a minimum diameter greaterthan the largest 1 pansion valve means for reducing the pressure of astream of the compressed and refrigerated gaseous mixture, a highpressure fractionating column, a low pressure fractionating column,means for dividing the expanded stream into a pair of substreams,conduit means for conducting one substream of the stream of the expandedmixture to the high pressure fractionating column as feed, meansassociated with the high pressure fractionating column for separatingthe feed'lnto a liquid product and a vapor product, means for condensingthe vapor product to form a liquefied product, second conduit means forconducting the liquid product of the high pressure fractionating columnto the low pressure fractionating column as feed, third conduit meansfor conducting the liquefied product from the condensing means to thelow pressure fractionating column as reflux, conduit means forconducting the other substream of the stream of expanded mixture to thelow pressure fractionating column as feed, and a fixed orificeinterposed in each of the second, third and fourth conduit means, theorifice in each conduit' means constituting the sole means forcontrolling the rate of liquid flow therethrough at any constantpressure difierential between the columns and the orifices in the secondand third conduit means being balanced against the orifice in the fourthconduit means to proportion the stream of expanded mixture between thepair of substreams.

7. Apparatus as claimed in claim 6 in which each oririce is in elongatedtubular form and of relatively small bore.

8(The method of separating a gaseous mixture in a fractionatingoperation including a high pressure fractionating zone and a lowpressure fractionating Zone, comprising the steps of expanding by meansof a controllable expansion valve a stream of gaseous mixture to befractionated, the stream of gaseous mixture being expanded to thepressure of the high pressure fractionating zone, dividing the expandedstream into a pair of substreams, introducing one of the substreams intothe high pressure fractionating zone in which a preliminaryfractionation takes place producing a liquid fraction and a gaseousfraction, liquefying a stream of gaseous fraction, expanding the streamsof liquefied gaseous fraction and iiquid fraction and the othersubstream of expanded gaseous mixture to the pressure of the lowpressure fractionating zone, introducing a stream of expanded liquefiedgaseous fraction into the low pressure fractionating zone as reflux,introducing a streamof expanded liquid fraction into the low pressurefractionating zone as feed, introducing the expanded other substream ofthe stream of expanded gaseous mixture into the low pressurefractionating zone as feed, and balancing the rate of flow of theliquefied gaseous fraction and the liquid fraction streams introducedinto the low pressure fractionating zone t aiust the rate of flow of theother substream of ,xpanded gaseous-mixture introduced into the lowpressure fractionatingzone to proportion the stream of expanded gaseousmixture between the pair of substreams.

9. The method of separating a gaseous mixture in a fractionatingoperation including a high pressure fractionating zone and a lowpressure fractionating zone,

comprising the steps of expanding without liquefaction by means of acontrollable expansion valve a stream of gaseous mixture to befractionated, the stream of gaseous mixture being expanded to thepressure of the high pressure fractionating zone, dividing the expandedstream into 8 a pair of substreams, introducing without a further expansion step one of the substreams into the high pressure fractionatingzone in which a preliminary fractionation takes place producing a liquidfraction and a gaseous fraction, liquefying a stream of the gaseousfraction, in-

troducing a stream of liquefied gaseous fraction into the low pressurefractionating zone as reflux, introducing a stream of liquid fractioninto the low pressure fractionating zone as feed, introducing the othersubstream of the expanded gaseous mixture into the low pressurefractionating zone as feed, expanding the streams of liquefied gaseousfraction and liquid fraction and the other substream of expanded gaseousmixture to the pressure of the low pressure fractionating zone beforeintroduction into the low pressure fractionating zone, and balancing therate of flow of the liquefied gaseous fraction and the liquid fractionstreams introduced into the low pressure fractionating zone against therate of flow of the other substream of expanded gaseous mixtureintroduced into the low pressure fractionating zone to proportion theream of expanded gaseous mixture between the pair of substreams.

10. The method of separating a gaseous mixture in a fractionatingoperation including ahigh pressure fractionating zone and a low pressurefractionating zone, comprising the steps of expanding by means of acontrollable expansion valve a stream of gaseous mixture to befractionated, the streamof gaseous mixture being expanded to thepressure of the high pressure fractionating zone, dividing the expandedstream into a pair of substreams, introducing one of the substreams intothe high pressure fractionating zone in which a preliminaryfractionation takes place producing a liquid fraction and a gaseousfraction, liquefying a stream of gaseous fraction, introducing throughconduit means a stream of liquefied gaseous fraction expanded to thepressure of the low pressure fractionating zone into the low pressurefractionating zone as reflux, introducing through conduit means a streamof liquid fraction expanded to the pressure of the low pressurefractionating zone into the low pressure fractionating zone as feed,introducing through conduit means the other substream of theexpandedgaseous mixture expanded to the pressure of the low pressurefractionating zone into the low pressure fractionating zone as feed,establishing fixed pressure drops of the streams of fluid passingthrough the conduit means and balancing the pressure drop of the streamof liquefied gaseous fraction and the stream of liquid fraction againstthe pressure drop of the other substream of the expanded gaseous mixtureto proportion the stream of expanded gaseous mixture between the pair ofsubstreams;

11. Apparatus for fractionating compressed and refrigerated gaseousmixtures comprising a controllable expansion valve means for reducingwithout liquefaction the pressure of a stream of the compressed andrefrigerated gaseous mixture, a high pressure fractionating column, alow pressure fractionating column, means for dividing the expandedstream into a pair of substreams,

first conduit means for conducting without further expansion onesubstream of the expanded mixture to 'the high pressure fractionatingcolumn as feed, means associated with the high pressure fractionatingcolumn for separating the feed into a liquid fraction and a vaporfraction, means for condensing a stream of vapor fraction from the highpressure fractionating column to form a stream of liquefied'fraction,'second conduit means for a means constituting the sole meansfor controlling the rate of liquid flow therethrough at anypredetermined pressure ditferential between the columns, and theorifices in the second and third conduit means being balanced againstthe orifice in the fourth conduit means to proportion the expandedstream of gaseous mixture between the pair of substreams.

References Cited in the file of this patent UNITED STATES PATENTS1,917,891 Levin July ll, 1933 10 Fraser July 31, Linde July 21, SchlittJuly 30, Ross June 23, Dennis Oct. 9, Van Nuys Oct. 15, Dennis Dec. 2,Dennis Dec. 30, Van Nuys Dec. 30, Latham Aug. 31,

1. IN AN AIR FRACTIONATING APPARATUS HAVING FRACTIONATING COLUMNSARRANGED TO OPERATE RESPECTIVELY AT RELATIVELY HIGH AND RELATIVELY LOWPRESSURE; MEANS FOR LIQUEFYING AT THE HIGHER PRESSURE A STREAM OFGASEOUS NITROGEN DRAWN FROM THE HIGH PRESSURE COLUMN; MEANS FOREXPANDING TO THE LOWER PRESSURE A STREAM OF CRUDE OXYGEN DRAWN FROM THEHIGH PRESSURE COLUMN; MEANS FOR SUBCOOLING SAID LIQUID NITROGEN STREAMBY HEAT INTERCHANGE WITH SAID EXPANDED CRUDE OXYGEN STREAM; MEANS FOREXPANDING SAID SUBCOOLED NITROGEN TO THE LOWER PRESSURE; TRANSFER MEANSINCLUDING A PATH THROUGH A LIQUID RECEIVING RESERVOIR COMMUNICATING WITHTHE LAST-NAMED EXPANSION MEANS AND THE LOW PRESSURE COLUMN FORDELIVERING EXPANDED SUBCOOLED NITROGEN TO THE LOW PRESSURE COLUMN, ANDVALVE CONTROLLED MEANS FOR WITHDRAWING DESIRED QUANTITIES OF LIQUIDNITROGEN FROM SAID RESERVOIR.